01 -
Heat butter and olive oil in a large skillet over medium-low heat. Add thinly sliced onions with a pinch of salt and cook slowly, stirring occasionally, for 30 to 40 minutes until golden brown and caramelized.
02 -
Divide ground beef into 8 equal portions and form thin patties. Season both sides generously with salt and pepper. Create a small indent in the center of each patty to prevent puffing during cooking.
03 -
Preheat a large skillet or grill pan over medium-high heat. Cook patties for 3 to 4 minutes per side until medium doneness. During the last minute, top each patty with cheese slices and cover to melt.
04 -
Split the brioche buns and toast the cut sides in a dry skillet or under the broiler until golden brown and crispy, approximately 1 to 2 minutes.
05 -
Place the bottom bun on a plate. Add one cheese-topped patty, followed by a generous layer of caramelized onions. Place second cheese-topped patty on top, add more onions, then crown with the top bun. Garnish with fresh chives and serve immediately.